Abrasion resistant coatings

ABSTRACT

A substrate is coated with a fluoropolymer non-stick coating comprising an undercoat and an overcoat, each containing fluoropolymer resin, said undercoat containing ceramic particles which extend from said undercoat, wherein said ceramic particles telegraph their presence through the overcoat, forming protrusions on the coating surface, to provide abrasion resistance to said coating by deflecting abrasion force away from the coating.

RELATED APPLICATION

Priority under 35 USC 119 is claimed of Provisional Application No.60/120,853, filed Feb. 19, 1999.

FIELD OF INVENTION

This invention relates to fluoropolymer coating compositions andsubstrates coated with these compositions which have improved abrasionresistance.

BACKGROUND OF THE INVENTION

Fluoropolymer resins, and especially perfluoropolymer resins, are knownfor their low surface energy and non-stick properties as well as thermaland chemical resistance. It has long been desirable to achieve longerwearing non-stick polymer coatings on metal substrates. Of particularconcern to achieving coated substrates with longer service life is thecoated substrate's ability to withstand abrasion as well as its scratchresistance. “Scratch” is related to plastic deformation of the coatingsuch as a cut from a knife or other metal tool. Abrasion refers to theamount of coating that is worn away as may occur by rubbing or sandingwherein the coating fibrillates and breaks away or shreds from thesurface. In damaging a coated substrate, scratch may be followed byabrasion, in that a knife which causes plastic deformation of thecoating, may also lead to the formation of fibrils which aresubsequently worn away.

The problem of durability of the non-stick coating has often been viewedas one of adhesion of the coating to the metal substrate. If the coatingis optimized for release so as to prevent food particles from stickingto it after cooking or to facilitate low friction sliding contact inother applications, almost by definition there will be difficulties ingetting non-stick coatings to adhere well to the substrate.

Generally in the art, adhesion has been achieved by roughening the metalsubstrate prior to application of the non-stick coating so thatmechanical bonding will assist chemical interaction of binders in aprimer layer in promoting adhesion. Typical roughening includesacid-etching, sanding, grit-blasting, brushing and baking a rough layerof glass, ceramic or enamel frit onto the substrate. Other means ofincreasing adhesion and hence durability have included arc spraying amechanically resistant layer of metallic materials onto a roughenedmetal substrate as disclosed in U.S. Pat. Nos. 5,411,771 (Tsai) and5,462,769 (Tsai). Roughening substrate or applying a mechanicallyresistant metallic layer to improve adherence adds additional cost tothe coating operation and in the case of chemical etching, there areadditional costs of disposing etchant materials.

Prior efforts at achieving scratch-resistant coatings have includedusing harder auxiliary heat resistant resins along with perfluorocarbonpolymers. Sometimes fillers such as mica and aluminum flake have beenused in attempt to improve scratch resistance as disclosed in U.S. Pat.Nos. 4,180,609 (Vassiliou) and 4,123,401 (Berghmans et al.). Improvedscratch resistance attributable to inorganic fillers and fillers of heatresistant polymers is disclosed in U.S. Pat. No. 5,106,682 (Matsushita).In U.S. Pat. No. 5,250,356 (Batzar), a multilayer system is disclosedwhich uses high build primer reinforced with small particle sizealuminum oxide, an aluminum oxide reinforced intermediate coat and aconventional topcoat which provides release and yet shows reducedscratching. The above references all rely on grit blasting or fritcoating of the aluminum substrate to achieve the proper adhesion.

All of the prior art solutions discussed above, while they may attemptto achieve longer life coatings by increasing adhesion or reducingscratch, do not address the mechanism of wear and how to deflectabrasive forces away from the coating surface.

SUMMARY OF THE INVENTION

The present invention addresses the need for a durable, non-stickcoating with superior abrasion resistance. Specifically the inventionprovides for a substrate coated with a non-stick coating which resistsabrasion force, the coating comprising a fluoropolymer resin containingceramic particles extending through the thickness of the coating todeflect the abrasion force away from the coating wherein the ratio ofthe thickness of the coating to the longest diameter of the ceramicparticles is in the range of 0.8-2.0.

Further, the invention provides for a substrate coated with a non-stickcoating which resists abrasion force, the coating comprising anundercoat and an overcoat, each containing fluoropolymer resin, theundercoat also containing ceramic particles extending from theundercoat, the overcoat telegraphing the particles extending from theundercoat through the thickness of the overcoat to deflect the abrasionforce away from the coating. Preferably, the ratio of the combinedthickness of the undercoat and the overcoat to the longest diameter ofthe ceramic particles is in the range of 0.8-2.0.

The invention further comprises a composition capable of forming anadherent coating to a smooth substrate and exhibiting abrasionresistance, comprising fluoropolymer, polymer binder and inorganicfiller film hardener, wherein at least 30 wt % of the film hardener iscomprised of large ceramic particles having an average particle size ofat least 14 micrometers with the amount of ceramic particles beingsufficient to provide at least 3 such particles per 1 cm length oftransverse cross section of coating formed from said composition.

In another embodiment, the invention comprises a primer compositioncapable of forming an adherent coating to a smooth substrate andexhibiting abrasion resistance, comprising fluoropolymer, polymer binderand inorganic filler film hardener, the fluoropolymer to polymer binderweight ratio being 0.5 to 2.0:1 and the filler film hardener tofluoropolymer weight ratio being at least 1.4:1. Preferably the fillerfilm hardener comprises large ceramic particles having an averageparticle size of at least 20 micrometers. More preferably the amount oflarge ceramic particles is sufficient to provide at least 3 suchparticles per 1 cm length of transverse cross section of coating formedfrom said primer composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a transverse cross section of a substratecoated with a non-stick composition of fluoropolymer containing ceramicparticles.

FIG. 2 is a scanning electron micrograph at 1000 magnification of atransverse cross section of a coated substrate showing a SiC particlesin the undercoat and a deflection point in the surface of the coating.

FIG. 3 is a graph that shows the relation of SiC particle size at aconstant load to abrasion resistance.

FIG. 4 is a graph that shows the relation of concentration of largeparticles of SiC to abrasion resistance.

DETAILED DESCRIPTION

The present invention achieves non-stick coating compositions withsuperior abrasion resistance by incorporating an inorganic filler filmhardener comprising large ceramic particles into fluoropolymer resincoating compositions. The large ceramic particles are contained incoating composition and when applied to a substrate, extend through thethickness of the coating telegraphing the particles so that abrasionforces are deflected away from the coating. Such deflection reduces theinstances of abrasive forces shredding the coating which would result inthe removal of coating. The invention is the recognition of the properbalance between using large, hard particles to deflect abrasive forces,while still retaining sufficient non-stick properties of the coatingsupplied by the fluoropolymer. By “large” it is meant that the particleis large when considering it to the total dry film thickness (dft) ofthe coating. The ratio of (a) total dry film thickness to (b) longestdiameter of the ceramic particles is in the range of 0.8-2.0. FIG. 1 isa schematic of a transverse cross section of a substrate 10, a non-stickcoating 12, and filler particles 13, 14, 15, 16, and 17. The figure isillustrative of particle size defmition. The arrow represented by “a” isa measure of total dry film thickness of a coating in the area where theparticle is located. The arrow represented by “b” is a measure of thelongest diameter of a particle. In examining the particles for a givencoating thickness as illustrated in FIG. 1, particles 13, 14, and 16have ratios within the defined range of this invention and “telegraph”through the thickness of the coating to produce deflection points 18,19, 20 in the surface of the coating. Particles that fall within thedefined ranged of the invention promote deflection points in the surfaceof the coating capable of withstanding abrasive forces. Particle 15 is“too small” to aid in abrasion resistance and thus no deflection pointis telegraphed to the surface of the coating. Particle 17 is “too large”and actually breaks through the coating surface reducing the non-stickand low friction properties desirable in fluoropolymer coatings.

a/b ratios for an average dft of 45 micrometers

a/b ratios for an average dft of 45 micrometers Particle 13, where b =35 micrometers: a/b = 1.3 - in the claimed range Particle 14, where b =56 micrometers: a/b = 0.8 - maximum large Particle 15, where b = 16micrometers: a/b = 2.8 - too small Particle 16, where b = 23micrometers: a/b = 2.0 - minimum large Particle 17, where b = 64micrometers: a/b = 0.7 - too large

A deflection point in the coating is caused by the presence of a largeparticle just beneath the surface of the coating which promotes theabrasion resistance. Theoretically, the ideal particle shape for theceramic particles would be a sphere where the a/b ratio is 1.1. Thiswould mean that a particle positioned on the substrate extends from thesubstrate through approximately 90% of the coating thickness, beingstill about 10% below the surface of the coating. However, the ceramicparticles used in this invention are generally not spherical, having alarge and small diameter. It is preferred that the particle beessentially surrounded by the non-stick coating and not protrude throughthe surface of the coating. According to this invention, for abrasionresistant coatings, the preferred number of particles in the range of0.8 to 2.0 is at least 3 per 1 cm length of a transverse cross sectionof a coated substrate.

It is also preferred that the ceramic particles have an aspect ratio ofnot greater than 2.5 and preferably not greater than 1.5. By aspectratio is meant a ratio of the longest diameter “b” of the particle tothe greatest distance of a dimension “s” measured perpendicular to thelongest diameter (major axis) of the particle (FIG. 1). The aspect ratiois a means of quantifying a preferred particle shape and orientation.Particles with a high aspect ratio are flat or elongated, unlike thepreferred particles of this invention, which are preferably morespherical and more closely approach an ideal aspect ratio of 1. Ifparticles in a coating on a substrate are small and have a high aspectratio, they may be oriented parallel to a substrate and will not be ableto deflect abrasive forces applied to a coated substrate. If particlesare large and have a high aspect ratio, they may be orientedperpendicular to a substrate and protrude through a coating. An abrasiveforce may push against the tops of such particles distorting a coatingand even pulling a particle from the coating, leaving a hole and causingthe coating to be more rapidly abraded.

b/s Ratios

Referring to FIG. 1, aspect ratios b/s for particles within the claimedrange of this invention are

Particle 13 b/s = 2.3 Particle 14 b/s = 2.3 Particle 16 b/s = 1.3

However, Particle 17 is considered “too large” for the coating systemillustrated in FIG. 1 and has a b/s ratio=2.6. Particle 15 is considered“too small” for the coating system illustrated in FIG. 1 and thereforeits aspect ratio is not of consequence.

In a multilayer coating system comprising a substrate coated with anundercoat and an overcoat each containing fluoropolymer resin, theceramic particles are preferably contained in the undercoat and extendfrom the undercoat through the thickness of the overcoat, telegraphingthe particles so that abrasion forces are deflected away from thecoating. By “undercoat” it is meant any coating under the surfacecoating which may be a primer coating or one or more intermediatecoatings containing particles defined by this invention. By “overcoat”it is meant one or more additional intermediate coatings or a topcoatwhich telegraph the particles extending from the undercoat through thethickness of the coating. The ratio of the combined thickness of theundercoat and the overcoat to the longest diameter of ceramic particlesis in the range of 0.8-2.0. The number of ceramic particles in theundercoat extending from the undercoat and telegraphing the particlesthrough the thickness of the undercoat to deflect abrasion force awayfrom the coating is at least 3 per 1 cm length, preferably at least 10per 1 cm length, more preferably at least 15 per 1 cm length, and mostpreferably at least 25 per 1 cm length. Any ceramic particles whichextend above the general plane of the undercoat are still essentiallysurrounded by or coated with the undercoat material.

In a single-coat system comprising a substrate coated with a non-stickcoating, the coating contains a fluoropolymer resin with ceramicparticles to deflect said abrasion force away from said coating whereinthe ratio of the thickness of the coating to the longest diameter ofsaid ceramic particles is in the range of 0.8-2.0. The number of ceramicparticles in the coating to deflect abrasion force away from the coatingis at least 3 per 1 cm length, preferably at least 10 per 1 cm length,more preferably at least 15 per 1 cm length, and most preferably atleast 25 per 1 cm length.

Fluoropolymer Resin

The fluoropolymer component of the non-stick coating composition of thisinvention is preferably polytetrafluoroethylene (PTFE) having a meltviscosity of at least 1×10⁸ Pa·s at 380° C. for simplicity informulating the composition and the fact that PTFE has the highest heatstability among the fluoropolymers. Such PTFE can also contain a smallamount of comonomer modifier which improves film-forming capabilityduring baking (fusing), such as perfluoroolefin, notablyhexafluoropropylene (HFP) or perfluoro(alkyl vinyl) ether, notablywherein the alkyl group contains 1 to 5 carbon atoms, withperfluoro(propyl vinyl ether) (PPVE) being preferred. The amount of suchmodifier will be insufficient to confer melt-fabricability to the PTFE,generally being no more than 0.5 mole %. The PTFE, also for simplicity,can have a single melt viscosity, usually at least 1×10⁹ Pa·s, but amixture of PTFEs having different melt viscosities can be used to formthe fluoropolymer component. Use of a single fluoropolymer in thecomposition, which is the preferred condition, means that thefluoropolymer has a single chemical identity and melt viscosity.

While PTFE is preferred, the fluoropolymer component can also bemelt-fabricable fluoropolymer, either combined (blended) with the PTFE,or in place thereof. Examples of such melt-fabricable fluoropolymersinclude copolymers of TFE and at least one fluorinated copolymerizablemonomer (comonomer) present in the polymer in sufficient amount toreduce the melting point of the copolymer substantially below that ofTFE homopolymer, polytetrafluoroethylene (PTFE), e.g., to a meltingtemperature no greater than 315° C. Preferred comonomers with TFEinclude the perfluorinated monomers such as perfluoroolefins having 3-6carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) wherein the alkylgroup contains 1-5 carbon atoms, especially 1-3 carbon atoms. Especiallypreferred comonomers include hexafluoropropylene (HFP), perfluoro(ethylvinyl ether) (PEVE), perfluoro(propyl vinyl ether) (PPVE) andperfluoro(methyl vinyl ether) (PMVE). Preferred TFE copolymers includeFEP (TFE/HFP copolymer), PFA (TFE/PAVE copolymer), TFE/HFP/PAVE whereinPAVE is PEVE and/or PPVE and MFA (TFE/PMVE/PAVE wherein the alkyl groupof PAVE has at least two carbon atoms). The molecular weight of themelt-fabricable tetrafluoroethylene copolymers is unimportant exceptthat it be sufficient to be film-forming and be able to sustain a moldedshape so as to have integrity in the primer application. Typically, themelt viscosity will be at least 1×10² Pa·s and may range up to about60-100×10³ Pa·s as determined at 372° C. according to ASTM D-1238.

The fluoropolymer component is generally commercially available as adispersion of the polymer in water, which is the preferred form for thecomposition of the invention for ease of application and environmentalacceptability. By “dispersion” is meant that the fluoropolymersparticles are stably dispersed in the aqueous medium, so that settlingof the particles does not occur within the time when the dispersion willbe used; this is achieved by the small size of the fluoropolymerparticles, typically on the order of 0.2 micrometers, and the use ofsurfactant in the aqueous dispersion by the dispersion manufacturer.Such dispersions can be obtained directly by the process known asdispersion polymerization, optionally followed by concentration and/orfurther addition of surfactant. In some cases it is desirable to includean organic liquid, such as N-methylpyrrolidone, butyrolactone, highboiling aromatic solvents, alcohols, mixtures thereof, among others inthe aqueous dispersions.

Alternatively, the fluoropolymer component may be a fluoropolymer powdersuch as PTFE micropowder. In which case, typically an organic liquid isused in order to achieve an intimate mixture of fluoropolymer andpolymer binder. The organic liquid may be chosen because a binderdissolves in that particular liquid. If the binder is not dissolvedwithin the liquid, then the binder can be finely divided and bedispersed with the fluoropolymer in the liquid. The resultant coatingcomposition can comprise fluoropolymer dispersed in organic liquid andpolymer binder, either dispersed in the liquid or dissolved in order toachieve the intimate mixture desired. The characteristics of the organicliquid will depend upon the identity of the polymer binder and whether asolution or dispersion thereof is desired. Examples of such liquidsinclude N-methylpyrrolidone, butyrolactone, high boiling aromaticsolvents, alcohols, mixtures thereof, among others. The amount of theorganic liquid will depend on the flow characteristics desired for theparticular coating operation.

Polymer Binder

A fluoropolymer composition of this invention preferably contains a heatresistant polymer binder. The binder component is composed of polymerwhich is film-forming upon heating to fusion and is also thermallystable. This component is well known in primer applications fornon-stick finishes, for adhering the fluoropolymer-containing primerlayer to substrates and for film-forming within and as part of a primerlayer. The fluoropolymer by itself has little to no adhesion to a smoothsubstrate. The binder is generally non-fluorine containing and yetadheres to the fluoropolymer. Preferred binders are those that aresoluble or solubilized in water or a mixture of water and organicsolvent for the binder, which solvent is miscible with water. Thissolubility aids in the blending of the binder with the fluorocarboncomponent in the aqueous dispersion form.

An example of the binder component is polyamic acid salt which convertsto polyamideimide (PAI) upon baking of the composition to form theprimer layer. This binder is preferred because in the fully imidizedform obtained by baking the polyamic acid salt, this binder has acontinuous service temperature in excess of 250° C. The polyamic acidsalt is generally available as polyamic acid having an inherentviscosity of at least 0.1 as measured as a 0.5 wt % solution inN,N-dimethylacetamide at 30° C. It is dissolved in a coalescing agentsuch as N-methylpyrrolidone, and a viscosity-reducing agent, such afurfuryl alcohol and reacted with tertiary amine, preferablytriethylamine, to form the salt, which is soluble in water, as describedin greater detail in U.S. Pat. No. 4,014,834 (Concannon). The resultantreaction medium containing the polyamic acid salt can then be blendedwith the fluoropolymer aqueous dispersion, and because the coalescingagent and viscosity-reducing agent are miscible in water, the blendingproduces a uniform coating composition. The blending can be achieved bysimple mixing of the liquids together without using excess agitation soas to avoid coagulation of the fluoropolymer aqueous dispersion. Otherbinders that can be used include polyether sulfone (PES) andpolyphenylene sulfide (PPS).

Whether the primer composition is applied as a liquid medium, whereinthe liquid is water and/or organic solvent, the adhesion propertiesdescribed above will manifest themselves upon drying and baking of theprimer layer together with baking of the next-applied layer offluoropolymer to form the non-stick coating of the substrate.

For simplicity, only one binder may be used to form the binder componentof the composition of the present invention. However, multiple bindersare also contemplated for use in this invention, especially when certainend-use properties are desired, such as flexibility, hardness, orcorrosion protection. Common combinations include PAI/PES, PAI/PPS andPES/PPS.

The proportion of fluoropolymer and binder, especially if thecomposition is used as a primer layer on a smooth substrate, ispreferably in the weight ratio of 0.5 to 2.0:1. The weight ratios offluoropolymer to binder disclosed herein are based on the weight ofthese components in the applied layer formed by baking the compositionafter application to its substrate. The baking drives off the volatilematerials present in the coating composition, including the salt moietyof the polyamic acid salt as the imide bonds are formed during baking.For convenience, the weight of binder, when it is polyamic acid saltwhich is converted to polyamideimide by the baking step, can be taken asthe weight of polyamic acid in the starting composition, whereby theweight ratio of fluoropolymer to binder can be determined from theamount of fluoropolymer and binder in the starting composition. When thecomposition of the invention is in the preferred aqueous dispersionform, these components will constitute about 5 to 50 wt % of the totaldispersion.

Inorganic Film Hardener

The inorganic filler film hardener component is one or more filler typematerials which are inert with respect to the other components of thecomposition and thermally stable at its eventual baking temperaturewhich fuses the fluoropolymer and binder. The film hardener is waterinsoluble so that it is typically uniformly dispersible but notdissolved in the aqueous dispersion form of the composition of theinvention. Typically, the filler film hardener of this inventionpreferably comprises large ceramic particles having an average particlesize of at least 14 micrometers, preferably at least 20 micrometers, andmore preferably at least 25 micrometers.

Further the ceramic particles of the inorganic film hardener preferablyhave a Knoop hardness of at least 1200 and more preferably of at least1500. Knoop hardness is a scale for describing the resistance of amaterial to indentation or scratching. Values for the hardness ofminerals and ceramics are listed in the Handbook of Chemistry, 77^(th)Edition, pp. 12-186, 187 based on reference material from Shackelfordand Alexander, CRC Materials Science and Engineering Handbook, CRCPress, Boca Raton Fla., 1991. The film hardener component impartsdurability to the non-stick fluoropolymer composition applied as acoating on a substrate by deflecting abrasive forces applied to thecoating surface and by resisting penetration of sharp objects that havepenetrated the fluoropolymer overcoat.

The ceramic particles of the inorganic film hardener preferably have anaspect ratio (as defined above) of not greater than 2.5, and morepreferably not greater than 1.5. Preferred particles of this invention,which are able to deflect abrasive forces applied to a coatingcontaining the particles, have an aspect ratio of not greater than 2.5and a size wherein the largest diameter of the particle is at least 50%of the coating thickness and does not exceed 25% of the coating filmthickness.

Preferably the coating composition comprises at least 30 wt % of fillerfilm hardener containing large ceramic particles that have an averageparticle size of at least 14 micrometers, preferably at least 20micrometers, and more preferably at least 25 micrometers. Preferably theamount of the large particles is sufficient to provide at least 3 suchparticles per 1 cm length of transverse cross section of coating formedfrom the composition.

As described above the average particle size measurement is typical ofparticle sizes of this invention, but the particle size of suitableceramic particles is a function of the ratio of the particle size to thetotal dry film thickness of the coating. The ratio of (a) total dry filmthickness to (b) longest diameter of the ceramic particles is in therange of 0.8-2.0. Thus for single coat systems or systems with low filmbuilds the average particle size needed for the invention will besmaller than those particles required for multi-coat systems or systemswith higher film builds. The ceramic particles contained in a coatingcomposition and applied to a substrate extend through the thickness ofthe coating telegraphing the particles so that abrasion forces aredeflected away from the coating.

Examples of inorganic filler film hardener include inorganic oxides,carbides, borides and nitrides having a Knoop hardness of at least 1200.Preferred are inorganic oxides, nitrides, borides and carbides ofzirconium, tantalum, titanium, tungsten, boron, aluminum and beryllium.Particularly preferred are silicon carbide and aluminum oxide. TypicalKnoop hardness values for preferred inorganic compositions are: zirconia(1200); aluminum nitride (1225); beryllia (1300); zirconium nitride(1510); zirconium boride (1560); titanium nitride (1770); tantalumcarbide (1800); tungsten carbide (1880); alumina (2025); zirconiumcarbide (2150); titanium carbide (2470); silicon carbide (2500);aluminum boride (2500); titanium boride (2850).

Other Fillers

In addition to the large particles of inorganic filler film hardener,the non-stick coating compositions of this invention may contain smallerparticles of inorganic filler film hardener as well as other fillermaterials having a Knoop hardness value of less than 1200. Preferablythe filler film hardener to fluoropolymer resin weight ratio is at least1.4:1. More preferably at least 30 wt % of the filler film hardener arelarge ceramic particles having an average particle size of at least 14micrometers, preferably at least 20 micrometers, and more preferably atleast 25 micrometers and the amount of the large particles is sufficientto provide at least 3 such particles per 1 cm length of transverse crosssection of coating formed from the composition.

Suitable additional fillers include small particles of aluminum oxide,calcined aluminum oxide, silicon carbide etc. as well as glass flake,glass bead, glass fiber, aluminum or zirconium silicate, mica, metalflake, metal fiber, fine ceramic powders, silicon dioxide, bariumsulfate, talc, etc.

Coating Application

The compositions of the present invention can be applied to substratesby conventional means. Spray and roller application are the mostconvenient application methods, depending on the substrate being coated.Other well-known coating methods including dipping and coil coating aresuitable. The non-stick coating compositions may be a single coat or amulti-coat system comprising an undercoat and an overcoat. The overcoatof one or more fluoropolymer-containing layers can be applied byconventional methods to the undercoat layer prior to its drying. Whenthe undercoat and overcoat layer compositions are aqueous dispersions,the overcoat composition can be applied to the undercoat layerpreferably after drying to touch. When the undercoat layer is made byapplying the composition from an organic solvent, and the next layer(intermediate coat or topcoat) is applied from an aqueous medium, theundercoat layer should be dried so that all water-incompatible solventis removed before application of such next layer.

A resultant composite structure can be baked to fuse all the coatings atthe same time to form a non-stick coating on the substrate. When thefluoropolymer is PTFE, a quick high bake temperature is preferred, e.g.,for 5 min. at a temperature starting at 800° F. (427° C.) and rising to825° F. (440° C.). When the fluoropolymer in the primer or the overcoatis a blend of PTFE and FEP, e.g., 50-70 wt % PTFE and 50-30 wt % FEP,the bake temperature may be reduced to 780° F. (415° C.), rising to 800°F. (427° C.) in 3 minutes (total bake time). The baked undercoat layerthickness is measured with film thickness instruments based on theeddy-current principle (ASTM B244) or magnetic induction principle (ASTMB499) and will generally be between 5-20 micrometers. The overcoat layerthickness will generally be from 10-25 micrometers (for both theintermediate coat layer and the topcoat layer).

In the resultant composite structure, the substrate can be of anymaterial which can withstand the bake temperature, such as metal andceramics, examples of which include aluminum, anodized aluminum,cold-rolled steel, stainless steel, enamel, glass, and pyroceram. Thesubstrate can be smooth, i.e. having a surface profile of less than 50microinches (1.25 micrometers) as measured by a model RT 60 surfacetester made by Alpa Co. of Milan, Italy, and needs to be clean. Forpyroceram and some glass, improved results are obtained by activation ofthe substrate surface such as by a slight chemical etch, which is notvisible to the naked eye, i.e., the surface is still smooth. Thesubstrate can also be chemically treated with an adhesion agent such asa mist coat of polyamic acid salt, such as disclosed in U.S. Pat. No.5,079,073 to Tannenbaum. When the undercoat layer is a primer, it can beconsidered as the first fluoropolymer-containing layer on the substrate,and preferably the primer layer is directly bonded to the substrate.

Products have non-stick finishes made using compositions of the presentinvention include cookware, bakeware, rice cookers and inserts therefor,water pots, iron sole plates, conveyors, chutes, roll surfaces, cuttingblades, etc.

TEST METHODS

Taber Abrasion Test

An abrasion test is conducted generally according to ASTM D4060 whereinthe surface of a film is subjected to the wear of two abrasive wheels ata known load. The weight loss and the dft loss are a measure of theresistance of a film to abrasion and is determined after a specifiednumber of cycles. The apparatus used is a Taber Abrasion Model 503, fromthe Taber Instrument Company. All Taber testing is done with CS17abrasive wheels/1 kg load/1000 cycles except where noted differently.The wheels are cleaned to refresh the abrasive surface every 250 cycles.

Sand Paper Abrasion Test (SPAT)

Samples of non-stick coated aluminum panels (10 cm×30 cm) are abradedwith squares (5 cm×5 cm) pieces of sandpaper. The sandpaper has a roughsurface and a-pressure adhesive coated soft side. For purposes of thetest, the soft side is adhered to a soft sponge (7×7×2.5 cm) leaving therough side of the sandpaper exposed. The rough side of the sandpaper isapplied against the non-stick coating under a constant specified load.The sandpaper is oscillated back and forth across a total length of 16.4cm at a frequency of 53 cycles per minute. After a specified number ofcycles, the sandpaper is replaced by a new piece of sandpaper. The filmthickness of the coating is measured initially and after a specifiednumber of cycles. The measurement is performed at the center of thetrack formed by the abrading sandpaper; i.e. approximately 8 cm fromboth sides. Abrasion is expressed by the loss of film thickness as afunction of the number of cycles.

Mechanical Tiger Paw Abrasion Test (MTP)

A coated substrate is continuously scratched with three point pen tipswhich are held by a weighted holder (400 g total weight) which rotatesthe pens against and around the surface of the coated substrate. Toaccelerate the failure of the entire thickness of the coating, i.e., therotation of the pens produces a continuous circular-shaped pathpenetrating the entire coating to reach the substrate, the substrate isheated at 200° C. during this scratch test, and the time to such failureis recorded. The longer the time to failure, the better the durabilityof the non-stick coating.

Mechanical Utensil Scratch Test

(MUST) Samples of non-stick coated aluminum substrates are tested forboth abrasion and scratch resistance by applying a triangular metallathe bit to the coated surface. The triangular lathe bit is a tungstencarbide turning insert, TNMG 322, commercially available from MSCIndustrial Supply Company, Plainview, N.Y. The coated substrate ismounted on a hot plate heated to a temperature of 400 ° F. (204° C.) tosimulate cooking conditions. The coated substrate is subjected to themovement of a reciprocating arm mounted with the triangular bit under aspecified load of 1.6 Kg. The bit is oscillated at a frequency of 1cycle per second across the non-stick surface creating a wear pattern 3mm×53 mm. The bit is permitted to oscillate until one observes the firstappearance of bare metal in the groove made by the bit. Upon thisobservation the test is stopped and the total number of cycles isrecorded. The test is repeated three times and the average number ofcycles to failure is recorded.

EXAMPLES

Fluoropolymer

PTFE micropowder: Zonyl® Fluoroadditive grade MP 1600, available fromthe DuPont Company, Wilmington, Del.

PTFE-1 dispersion: TFE fluoropolymer resin dispersion with standardspecific gravity (SSG) 2.25 measured according to ASTM D4895 and rawdispersion particle size (RDPS) 0.25-0.28 micrometers.

PTFE-2 dispersion: DuPont TFE fluoropolymer resin dispersion grade 30,available from the DuPont Company, Wilmington, Del.

FEP dispersion: TFE/HFP fluoropolymer resin dispersion with a solidscontent of from 54.5-56.5 wt.% and RDPS of from 150-210 nanometers, theresin having an HFP content of from 9.3-12.4 wt. % and a melt flow rateof 11.8-21.3 measured at 372° C. by the method of ASTM D-1238 modifiedas described in U.S. Pat. No. 4,380,618.

PFA dispersion: DuPont PFA fluoropolymer resin dispersion grade 335,available from the DuPont Company, Wilmington, Del.

Polymer binder

Two polyamideimide resins are used:

PAI-1 (in Example 1) is a 36 wt % solution of PAI resin in an NMP-basedsolvent containing naphtha and butyl alcohol in ratioNMP/naphtha/alcohol=40.5/21.5/2.0 (grade PD-10629, Phelps-Dodge MagnetWire Co.).

PAI-2 (in Example 3) is Torlon® AI-10 poly(amide-imide) (Amoco ChemicalsCorp.), a solid resin (which can be reverted to polyamic salt)containing 6-8% of residual NMP.

Polyamic acid salt is generally available as polyamic acid having aninherent viscosity of at least 0.1 as measured as a 0.5 wt % solution inN,N-dimethylacetamide at 30° C. It is dissolved in a coalescing agentsuch as N-methyl pyrrolidone, and a viscosity reducing agent, such asfurfuryl alcohol and reacted with tertiary amine, preferably triethylamine to form the salt which is soluble in water, as described ingreater detail in U.S. Pat. No. 4,014,834 (Concannon).

PPS Polyphenylene Sulfide Resin Ryton VI from Phillips Petroleum

Inorganic film hardener

Silicon carbide supplied by Elektroschmelzwerk Kempten GmbH (ESK),Munich Germany

P 1200 = 15.3 ± 1 micrometers average particle size P 600 = 25.8 ± 1micrometers average particle size P 400 = 35.0 ± 1.5 micrometers averageparticle size P 320 = 46.2 ± 1.5 micrometers average particle size

The average particle size is measured by sedimentation usingFEPA-Standard-43-GB 1984R 1993 resp. ISO 6344 according to informationprovided by the supplier.

Aluminum oxide supplied by Universal Abrasives, Stafford, England.

F 1200 3 micrometers average particle size F 500 12.8 micrometersaverage particle size F 360 22.8 micrometers average particle size F 24044.5 micrometers average particle size

The average particle size is measured by sedimentation usingFEPA-Standard-42-GB-1984 according to information provided by thesupplier.

Example 1

Single-coat system, silicon carbide

A non-stick coating composition comprising PTFE resin, polyamide imidebinder and solvent is prepared according to the composition in Table 1and to 100 grams of this composition, varying amounts of three grades ofsilicon carbide having different average particle sizes and a Knoophardness of 2500-2900 are added as shown in Table 2.

TABLE 1 Single-Coat Ingredients Weight Percent PTFE micropowder 11.050PAI-1 12.056 Melamine/formaldehyde resin 1.839 Carbon black pigment1.660 N-butanol 2.058 Methyl isobutyl ketone 17.530N-methyl-2-pyrrolidone 46.607 Naphtha 7.200 Total 100.000

A single layer of the coating composition is applied to cold rolledcarbon steel substrates which are smooth, having been treated only bywashing to remove grease but have not been mechanically roughened. Thecoating is applied by spray coating. The coating is baked at atemperature of 350° C., unless otherwise specified. The dry filmthickness of the coating is measured and listed Table 2. The coatedsubstrates are subjected to the Taber abrasion test as described aboveand the % abrasion (i.e., % lost film thickness divided by total dryfilm thickness initial×100) is listed in Table 2. All Taber testing iswith CS17/1 kg/1000 cycles except where noted differently. The coatedsubstrates of examples 1-3, 1-4, 1-6, and 1-7 are sectioned and viewedwith a scanning electron microscope (SEM).

TABLE 2 Abrasion Resistance, Single-Coat SiC bake dft * Gradetemperature initial Sample (g) ESK ° C. microns 1-1 0 350 24.6 1-2 3.0P1200 350 26.8 1-3 4.5 P1200 300 38.9 1-4 3.0 P600 350 19.5 1-5 3.0 P600350 29.2 1-6 3.0 P600 350 36.1 1-7 5.0 P600 350 59.7 1-8 5.0 P600 35026.8 1-9 5.0 P400 350 31.6 1-10 5.0 P400 350 47.1 * SiC (g) = gramsadded to 100 grams fluoropolymer composition listed in Table 1 (whichhas a solids of 25 wt % - formulation see above).

TABLE 2 Abrasion Resistance, Single-Coat SiC bake dft * Gradetemperature initial Sample (g) ESK ° C. microns 1-1 0 350 24.6 1-2 3.0P1200 350 26.8 1-3 4.5 P1200 300 38.9 1-4 3.0 P600 350 19.5 1-5 3.0 P600350 29.2 1-6 3.0 P600 350 36.1 1-7 5.0 P600 350 59.7 1-8 5.0 P600 35026.8 1-9 5.0 P400 350 31.6 1-10 5.0 P400 350 47.1 * SiC (g) = gramsadded to 100 grams fluoropolymer composition listed in Table 1 (whichhas a solids of 25 wt % - formulation see above).

As shown in example 1-1, with no addition of SiC, all the coating isworn and the abrasion is 100%. As shown in examples 1-2 to 1-10 theaddition of 3-5% SiC reduces the wear significantly with the percentabrasion varying from 4 to 61%. The lower abrasion performance (61%)shown in example 1-9 suggests that the a/b ratio is less than 0.8because the P-400 grade has about 3% of the particles in the range of48.2-77 micrometers (supplier information) and such particles are toolarge for an average film thickness of 31.6 micrometers. Particles ofthis size protrude through the coating surface and detract from thedesired non-stick properties of the composition.

The best abrasion result is obtained by the addition of 3% SiC type P600having an average particle size of 25.8±1 micrometers and a/b ratio inthe range of 1.0 or lower, i.e., the size of the SiC particles are aboutthe same or higher than the average film thickness. Although theabrasion resistance of such film is excellent, the film texture may besomewhat rough and may affect other properties such as release or gloss.As previously described, for a satisfactory system, a balance betweenabrasion resistance and release must be achieved.

Examples 1-3, 1-4, 1-6, and 1-7 show the presence of “large” particlesas defined by the a/b ratio falling in the range 0.8-2.0. The a/b ratiofor several particles in a cross section is listed. The number ofdeflection points in the surface of the coating was also higher than 3points/cm of cross section. For the sample 1-3 there are about 65 largeparticles per 1 cm length of a transverse cross section of the coating(i.e., 65 particles wherein the a/b ratio is in the range of 0.8-2.0).

Example 1 shows that the presence of “large” SiC particles improvesabrasion resistance of a single coat system. However, smaller particleswith ratios a/b of more than 2 may also be beneficial for the abrasionresistance in a single coat system because some of the smaller particlesmay reside closer to the surface of the coating than to the substrate,promoting additional deflection points in the coating surface.

Example 2

Multi-coat System, Aluminum Oxide in Primer

A 3-coat system of primer/intermediate coat/topcoat is applied toaluminum substrates by roller coating, i.e., where the coating isapplied to the substrate by a series of rollers. The substrates aresmooth, being washed to remove grease but not being mechanicallyroughened.

A primer composition as described in Table 3 promotes adhesion to themetal substrate and is applied at an average film thickness of 3micrometers.

TABLE 3 Primer Composition Ingredient Weight Percent Carbon BlackPigment 1.83 Aluminum Silicate Extender 0.92 “Ludox ™” sodium stabilizedColloidal 2.13 Silica from Du Pont PTFE-1 (MV 10¹⁰ Pa Sec) 8.61 FEP (MV2-4x10³ Pa Sec) 5.74 PAI-2 4.79 Sodium Polynaphthalenesulfonate 0.26Anionic Surfactant Surfynol 440 Nonionic Surfactant from 0.26 AirProducts Deionized Water 65.74 Octylphenolpolyethoxy nonionic surfactant0.31 Diethylethanol Amine 0.68 Triethylamine 1.35 Furfuryl Alcohol 3.72N-Methylpyrrolidone 3.14 Nonylphenolpolyethoxy nonionic surfactant 0.52Total 100

The composition for the intermediate coat contains PTFE, polymer binderand about 15 wt % fused alumina having a Knoop hardness of ˜2100 forexamples 2-2 to 2-4 is listed in Table 4 below. For comparison, theintermediate coating composition of example 2-1 contains no aluminumoxide. The intermediate coating compositions in examples 2-2 to 2-4 varyby the grade of alumina added, each grade varying in the averageparticle size as shown in Table 6. In example 2-2, particles of F-1200(3 micrometers average particle size) are added to the intermediatecoating composition. In example 2-3 a blend of two different particlesizes of alumina (F500 and F360 in a 33/66 ratio) are added. In example4, particles of F-240 (44.5 micrometers average particle size) areadded. The role of the intermediate coat is to promote adhesion andflexibility of the coating and is applied at an average film thicknessof 5-8 micrometers. The intermediate functions as the undercoat.

TABLE 4 Intermediate Composition Description Weight Percent Titaniumdioxide 6.21 Carbon black 2.26 Aluminum silicate 1.13 Barium sulfate3.66 Fused alumina 14.69 PTFE-1 24.02 Acrylic polymer 1.74 Polyphenylenesulfide 3.66 Surfynol 440 surfactant 0.29 Sodiumpolynaphthalenesulfonate 0.37 surfactant Octylphenolpolyethoxy 0.73nonionic surfactant Nonylphenolpolyethoxy 1.44 nonionic surfactantButylglycol 0.73 Water 37.09 Triethanolamine 1.98 Total 100 % weightsolids = 56% P/B = 99.6 (prime pigment = 30.2; extender = 36.21) Binder= 3.66 Fluoropolymer = 24.02 F/B = 6.6

The topcoat composition of composition as described in Table 5 belowprovides the non-stick (release) property and is applied at about 15micrometers. The topcoat functions as the overcoat.

TABLE 5 Topcoat Ingredients Weight Percent “Afflair” 153 Titaniumdioxide 4.92 Coated Mica flake from Merck Carbon black pigment 0.26Aluminum Silicate extender 0.13 PTFE-1 40.65 Sodiumpolynaphthalenesulfonate 0.04 surfactant Bevaloid 680 anti form agentfrom 0.35 BELALOID Water 41.64 Triethenolamine 6.99 Acrysol RM5 acrylicthickening 2.51 agent from Rohm and Haas Nonylphenylpolyethoxy 2.51non-ionic surfactant Total 100

The roller coat application technique is characterized by the formationof chicken tracks (irregular flow of the film). As a consequence of thechicken tracks the film build can vary between 7 and 70 micrometers (inthe extreme cases) with an average film build of 20-30 microns.

The multiple layers of coating system are applied sequentiallywet-on-wet with minimal drying and no curing between coats, then thecoated system is cured such as at about 400° C. for a minimum of threeminutes. The multi-coat system has a dry film thickness averaging 25micrometers. The substrates are subjected to the SPAT abrasion test asdescribed above. The SPAT abrasion test is performed as described in thetest methods above. The sandpaper is aluminum oxide P-220, STIKIT™ 255RD 800 B from the 3M Company, having an aluminum grain size of 55micrometers average. The specified load is 1.250 Kg. The test is run for400 cycles changing the abrasive paper every 100 cycles.

The results of this abrasion test are illustrated in Table 6. The coatedsubstrates are sectioned and viewed with a scanning electron microscope(SEM) to determine the a/b ratio (i.e., the ratio of the coatingthickness in relation to the longest dimension of a particle) and thenumber of particles with an a/b ratio of 0.8-2.0 per 1 cm length of atransverse cross section of the coating.

TABLE 6 Abrasion Resistance, Multi-Coat Abrasion Grade 400 CyclesDeflection Example Alumina* (breakthrough) Points/cm a/b 2-1 NoneIntense wear none — 2-2 F1200 Medium wear none 3.5, 5.9 2-3 F500 & F360Little wear 10 1.3, 1.7, 2.9 (33/66) 2-4 F240 None  7 1.5 *Fused Aluminafrom Universal Abrasives

The results in Table 6 show that as the alumina particle size increases,the abrasion resistance improves. As shown in example 2-1, with noaddition of aluminum oxide, the substrate is intensely worn revealingbare metal. For example 2-2, a medium amount of wear is visiblesuggesting that F 1200 particles (3 micrometers average particle size)are too small for a this multi-coat system. Example 2-3 with largerF500/F360 alumina particles withstands the abrasion of the SPAT testwith little wear and no wear is noticeable in example 2-4 when evenlarger F240 alumina particles are added.

For example 2-3, the SEM shows there are 10 large particles per 1 cmlength of a transverse cross section of the coating (10 particles withinthe range a/b=0.8-2.0) resulting in 10 deflection points on the coatingsurface. For example 2-4, the SEM there are 7 large particles per 1 cmlength of a transverse cross section of the coating (7 particles withinthe range a/b=0.8-2.0) resulting in 7 deflection points on the coatingsurface. For examples 2-1 with no alumina and 2-2 with particles ofalumina having a small average particle size and a/b ratios >2, thereare no deflection points in the surface of the coating consistent withpoor performance in the abrasion tests.

For multiple coat systems such as described in Example 2, the largeparticles are added to the undercoat only. They must be large enough toextend from the undercoat and be telegraphed through the thickness ofthe overcoat to create deflection points in the surface of the coating.Therefore, in multi-coat system, small particles in the intermediatecoat are unlikely to cause any additional abrasion improvement as can beseen in the one coat system of Example 1.

Example 3

Multi-coat System, Silicon Carbide in Primer

A 3-coat non-stick system of primer/intermediate coat/topcoat with SiCparticles in the primer is sprayed onto a smooth aluminum substrate,which has been treated only by washing to remove grease but notmechanically roughened. The SiC powder is a blend of three gradesP320/P400/P600 at the weight ratio of 20/40/40. The average particlesizes are as specified above. The composition of the primer is listed inTable 7. The primer functions as the undercoat and is applied on asmooth aluminum substrate and dried. The surface texture looks likesandpaper.

The intermediate coat is then sprayed over the dried primer. The topcoatis applied wet on wet to the intermediate coat. The compositions of theintermediate coat and the topcoat are listed in Tables 8 and 9respectively. The intermediate coat and the topcoat function asovercoats. The coating is cured by baking at a temperature of 430° C.

It is important to apply the primer/intermediate coat/topcoat at acontrolled film build, respectively 15-20/15/5-10 micrometers, becausethe surface of the primer is very rough and the valleys are filled bythe intermediate coat and topcoat. The large SiC particles extend fromthe primer (undercoat) through the thickness of the overcoat lying justbelow the top in order to create deflection points in the surface of thecoating. These large particles promote deflection points that are neededto resist abrasion.

The coated substrate is subjected to the MTP abrasion test, SPATabrasion test, and the MUST scratch and abrasion test as describedabove. The coated substrate is also sectioned and viewed with a scanningelectron microscope (SEM).

In the SPAT test, the specified load is 4.211 Kg and the sandpaper isaluminum oxide P320 (45 micrometers average grain size), type RDB 800Bfrom the 3M Company. The sandpaper is renewed every 50 cycles. The filmthickness is measured initially and after every 50 cycles.

TABLE 7 Primer Composition Ingredients Weight Percent PAI-1 4.28 Water59.35 Furfuryl Alcohol 3.30 Diethylethanolamine 0.60 Triethylamine 1.21Triethanolamine 0.20 N-Methylpyrrolidone 2.81 Furfuryl Alcohol 1.49Surfynol 440 surfactant 0.22 SiC P400 3.30 SiC P600 3.30 SiC P320 1.66PTFE-2 (solids in aqueous dispersion) 3.86 Alkylphenylethoxy surfactant1.59 FEP (solids in aqueous dispersion) 2.65 Ludox AM polysilicate 0.87Utramarine blue pigment 1.63 Carbon black pigment 0.28 Alumina 0.35-0.50micrometers 7.40 Total 100 % solids = 30.4 P/B = 142% Density = 1.21Vol. Sol. = 15.16%

TABLE 8 Intermediate Coat Ingredients Weight Percent PTFE-2 (solids inaqueous dispersion) 33.80 Nonylphenolpolyethoxy nonionic surfactant 3.38Water 34.82 PFA (solids in aqueous dispersion) 6.10Octylphenolpolyethoxy nonionic surfactant 2.03 Mica Iriodin 153 fromMERCK 1.00 Ultramarine blue pigment 0.52 Alumina 0.35-0.50 micrometers2.39 Triethanolamine 5.87 Cerium octoate 0.57 Oleic acid 1.21Butylcarbitol 1.52 Solvesso 100 hydrocarbon 1.90 Acrylic resin 4.89Total 100

TABLE 8 Intermediate Coat Ingredients Weight Percent PTFE-2 (solids inaqueous dispersion) 33.80 Nonylphenolpolyethoxy nonionic surfactant 3.38Water 34.82 PFA (solids in aqueous dispersion) 6.10Octylphenolpolyethoxy nonionic surfactant 2.03 Mica Iriodin 153 fromMERCK 1.00 Ultramarine blue pigment 0.52 Alumina 0.35-0.50 micrometers2.39 Triethanolamine 5.87 Cerium octoate 0.57 Oleic acid 1.21Butylcarbitol 1.52 Solvesso 100 hydrocarbon 1.90 Acrylic resin 4.89Total 100

A SEM micrograph of the cross section of the multi-layer coating isshown in FIG. 2. Because of the presence of large particles in thecoating, at each particle area where the ratio atb is in the range0.8-2.0, there is a deflection point. Particle 21 has an aspect ratio(b/s) of 1.4. Particle 21 is shown with an a/b ratio of 1.0 and adeflection point in the surface of the coating is shown at 22.

The coating of this example 3 has about 80 deflection points per 1 cmtransverse cross section. The coating resists for at least 3 hours theMTP abrasion test as described above. In comparison, an abrasion patternis obtained after only from 90-120 minutes with a commercial multi-coatsystem similar to that described in U.S. Pat. No. 5,160,791 Table 1 thathas no silicon carbide in it.

Similar results are obtained with the SPAT test using P1200 aluminasandpaper. After 3000 cycles the coating reinforced with SiC shows verylittle visible signs of abrasion and only a few microns of filmthickness loss. In comparison, a multi-coat system that is similar tothat described in U.S. Pat. No. 5,160,791 Table 1 that has no siliconcarbide in it, is a total abrasion failure, worn through to the metalafter the same number of cycles.

Substrates prepared per this example are also subjected to the MUST testas described above. The test has the attribute of performing acombination of abrasion and scratch testing on a substrate sample. Usingthe weighted oscillating triangular metal lathe bit, the non-sticksurface of a substrate sample is subjected to a series of three tests todetermine number of cycles until bare metal is exposed. For this sample,the test results show that bare metal is exposed after 303, 334, 265cycles respectively with an average of 301 cycles. In comparison, testresults for a commercial multi-coat system similar to that described inU.S. Pat. No. 5,160,791 Table 1 that has no silicon carbide in it, showthat bare metal is exposed after 135, 135,135 cycles respectively withan average of 135 cycles.

Example 4

Similar to Example 3, a series of smooth aluminum test panels are coated3-coat non-stick system of primer/intermediate coat/topcoat. The primercomposition on one panel contains no SiC particles in it. The otherpanels have primers each with 8.3 wt % SiC particles of a differentparticle size (3 micrometers avg., 15 micrometers, avg., >25 (blend asshown in Table 7) micrometers avg., respectively). All panels areovercoated with an intermediate coat and topcoat as described in Example3. The abrasion resistance of the coating is tested using the SPAT test,sanding with P320 alumina sandpaper under a load of 4.211 Kg. After each50 cycles the sandpaper is renewed and the film build is measured. Theresults are shown in FIG. 3. FIG. 3 is a graph that shows the relationof SiC particle size at a constant load of 8.3% by weight in the primerto abrasion resistance. The dry film thickness (dft) is plotted againstthe number of cycles of abrasion to determine the amount of film loss.With a multi-coat system having small particles (3 microns) in theprimer, the loss in film build is almost the same as for the primerwithout any SiC. With large SiC particles (>25 microns) in the primerthe abrasion resistance is much improved. Intermediate results areobtained with 15 microns SiC particles size.

Example 5

Similar to Example 3, a series of smooth aluminum test panels are coated3-coat non-stick system of primer/intermediate coat/topcoat. The primercomposition on panels vary in the amount SiC particles. The SiC powderin all primers is a blend of three grades P320/P400/P600 at the weightratio of 20/40/40. All panels are overcoated with an intermediate coatand topcoat as described in Example 3.

The abrasion resistance of the coating is tested using the SPAT test,sanding with P320 Alumina sandpaper under a load of 4.221 Kg. After each50 cycles the sandpaper is renewed and the film build is measured. Theresults are shown in FIG. 4. FIG. 4 is a graph that shows the relationof % weight SiC to the abrasion resistance (loss in film build). The dryfilm thickness (dft) is plotted against the number of cycles of abrasionto determine the amount of film loss. For greater amounts of largeparticle SiC there is less film loss from abrasion.

The number of deflection points are also measured by SEM examination ofthe film cross section for each sample tested for abrasion. A higherconcentration of ceramic results in a higher number of deflection pointsin the coating surface. The results are shown in the Table 10 below.

TABLE 10 SiC Concentration Number of % SiC by Weight Deflection Pointsin Primer per cm 0 0 1 3 3 10 6 19 8.3 28

The results clearly show that the number of deflection points increaseswith the concentration of the filler and promotes better abrasionresistance. Abrasion resistance is achieved with at least 3 deflectionpoints per cm of transverse cross section of the coating.

What is claimed is:
 1. A substrate coated with a non-stick coating whichresists abrasion force, said coating comprising an undercoat and anovercoat, each containing fluoropolymer resin, said undercoat alsocontaining ceramic particles extending from said undercoat, saidovercoat telegraphing said particles extending from said undercoatthrough the thickness of said overcoat to deflect said abrasion forceaway from said coating.
 2. The coated substrate of claim 1 wherein theratio of the combined thickness of said undercoat and said overcoat tothe longest diameter of said ceramic particles is in the range of0.8-2.0.
 3. The coated substrate of claim 1 wherein said ceramicparticles have a Knoop hardness of at least
 1200. 4. The coatedsubstrate of claim 3 wherein said ceramic particles have an aspect ratioof not greater than 2.5.
 5. The coated substrate of claim 3 wherein saidceramic particles are selected from a group consisting of inorganicnitrides, carbides, borides and oxides.
 6. The substrate of claim 1wherein said ceramic particles extending above the plane of saidundercoat are essentially surrounded by said undercoat.
 7. The coatedsubstrate of claim 1 wherein said overcoat comprises an intermediatecoat and a topcoat.
 8. The coated substrate of claim 1 wherein saidundercoat is a primer on said substrate.
 9. The coated substrate ofclaim 1 wherein said substrate prior to coating is smooth.
 10. Thecoated substrate of claim 1 wherein the number of said particles in saidundercoat is at least 3 per 1 cm length of a transverse cross section ofsaid coating.
 11. The coated substrate of claim 1 wherein said undercoatcontains at least one heat resistant polymer binder.
 12. A process forcoating the substrate of claim 1 wherein the undercoat and the overcoatare applied to the substrate without completely drying one coatingbefore applying the next, and the non-stick coating is formed by heatingto a temperature of at least 350° C.
 13. The coated substrate of claim 1in the form of cookware or bakeware.